1. Field of the Invention
The present invention concerns a method and an apparatus for image support of an operative procedure implemented with a medical instrument.
2. Description of the Prior Art
In the implementation of an operative procedure on a living subject, it is known to support the surgeon in the guiding of the medical instrument with optical image information. For this, an image of the instrument is mixed into a 2D or 3D image (acquired, for example, with an x-ray apparatus) of the treatment area of the subject.
In order to enable the implementation of such an intraoperative navigation, also known as fluoronavigation, a 2D x-ray image, for example of the treatment area, can be continuously created during the operative procedure, such that the instrument is imaged by the x-ray radiation together with the treatment area. Due to the high radiation stress with such a continual x-ray exposure, this is disadvantageous both for the treated individual and for the treatment personnel. To prevent such a high radiation exposure, it is known to temporally decouple the x-ray image generation and the operative procedure and to navigate the surgical instrument by means of a camera after a successful x-ray image generation, with an image of the instrument being mixed with correct position in real time into an existing (i.e. previously acquired) x-ray image. For this purpose, it is necessary to determine both the position of the instrument in a reference system and the position of this reference system relative to the image coordinates of the x-ray image.
The x-ray apparatus normally used for intraoperative navigation is in many cases a C-arm x-ray apparatus, in which the x-ray source and the x-ray receiver are mounted opposite one another on a C-arm that is pivotable around two axes relative to one another, in order to be able to generate x-ray images from different projection directions.
For the positionally precise mixing of an instrument, whose spatial position is known in a reference system, into a 2D x-ray image, precise knowledge of the spatial position of the projection cone belonging to this 2D image in this reference system is necessary. Ideally, the position of this projection cone is already known when the spatial position of the image plane, i.e. the entrance surface of the x-ray receiver, is fixed, such that it should be sufficient to determine the position of the x-ray receiver in the reference system in which the position of the instrument is also detected. In practice, however, it has proven to be the case that determination of the position of the x-ray receiver alone is not sufficient in order to be able to safely establish the projection cone, since this does not always exhibit the same relative position relative to the x-ray receiver due to the unavoidable deflection (distortion) of the C-arm dependent on the position of the x-ray axis.
In German OS 199 17 867, it is proposed to arrange on the x-ray receiver a reference structure that, in addition to optical position markers for camera-aided navigation, also has x-ray markers that lie in the beam path of the x-ray radiation and are imaged in the x-ray image. The precise position of the projection cone can be calculated and eventual distortions can also be eliminated by calculation from the position of the x-ray markers visible in the x-ray image.
The x-ray markers that are visible in the x-ray image have the disadvantage that they shadow a part of the x-ray image and in this manner are unavoidably associated with an information loss. This information loss cannot be completely prevented even when these x-ray markers are eliminated from the x-ray image by digital image processing.